1. Technical Field
The present disclosure relates to a method for soldering a cap to a support layer, and in particular for forming a package for a device manufactured by means of electronic technologies.
2. Description of the Related Art
As is known, in the field of the electronic packaging, caps are soldered to support layers (also known as substrates), so as to form cavities wherein electronic devices are hosted.
As an example, FIG. 1 shows a package 1 formed by a support layer 2 and a cap 3, the package 1 defining a cavity 6; an integrated electronic device 8, as an example formed by a Micro Electro-Mechanical System (MEMS), is located within the cavity 6. In general, in case of a MEMS, the integrated electronic device 8 comprises a first die (not shown), typically known as a sensor die and includes a micromechanical detection structure, and second die (not shown), typically known as an application-specific integrated circuit (ASIC) die and includes a related electronic interface. This first and second die may be stacked and are connected by means of suitable electrical connections in the form of wires designed to electrically connect the sensor die to the ASIC die, and the ASIC die to the support layer 2.
Irrespective of the details of the integrated electronic device 8, the support layer 2 has an internal surface 2a, on which the integrated electronic device 8 rests, and an external surface 2b, which carries suitable electrical connection elements 12 to the outside of the package 1, in the form of “balls” or “bumps” or “lands”.
Furthermore, the support layer 2 is usually made of a multi-layer structure, composed of several layers of conductive material separated via dielectric layers; electrical traces (not shown) and vias 10 are provided through the support layer 2, so as to electrically connect the integrated electronic device 8 to the electrical connection elements 12.
The cap 3 comprises a core 4 and a coating layer 5. The core 4 is generally of brass and is entirely coated by the coating layer 5, this latter being usually made up of tin or alloys such as a NiAu alloy. Furthermore, the cap 3 is soldered to the support layer 2 by means of the so-called conventional solder reflow technique. Therefore, the cap 3 is made integral with the support layer 2 by means of a solder joint 16, which defines a sealing ring interposed between the support layer 2 and the cap 3.
The coating layer 5 is made up of a so-called “wettable material”, which, upon contacting a solder paste and following a thermal treatment, can form an intermetallic compound with the solder contained in the solder paste.
In practice, the entire surface of the core 4 is covered with a solderable material. Therefore, as shown in FIG. 2, it may happen that, at the end of the soldering process, the solder extends not only in the gap G (FIG. 3) between the support layer 2 and the cap 3, but also along a portion of the inner wall of the cap 3, defined by a corresponding portion of the coating layer 5; in particular, the solder may extend along a vertical portion of the inner wall of the cap 3, orthogonally oriented with respect to the support layer 2.
The volume of solder which extends along the inner wall of the cap 3 is taken away from the gap G between the support layer 2 and the cap 3, thereby leading to the formation of voids V within the solder joint 16, as shown in FIG. 3.
In practice, because of the above mentioned voids V, it may happen that the package 1 is not fully sealed. Similarly, it may happen that the package 1 becomes unsealed on customer site, as an example because of slight mechanical shocks or during soldering of the package on the customer board.